Here's the history of Purple Martin nesting numbers in the county since the late 1970s to the present, from "Birds of Santa Barbara County, California":
Purple Martins nest in cavities in sycamores and conifers in Districts I and (formerly?) M. The only known breeding sites at present are at Nojoqui Falls County Park and in the nearby Solvang/Alisal Ranch area in the Santa Ynez Valley. Nojoqui Park is a persistent nesting location (see below) with a population of 10–15 individuals in the 1980s through 1993 (high count: 17 in July 1983). More recently, the typical counts at Nojoqui Falls Park have been between 6–12 individuals, with a high of 15 birds (including juveniles) 22 June 2007 and 12 August 2014, 21–30 birds between 22 June–22 July 2013, up to 17 adults during April and 6 nests with 17 young during July 2016, 24 birds there 25 April 2017, 25 on 18 July 2018, but only 9+ birds and up to 5 nests on 9 June 2020. The maximum count at Solvang/Alisal Ranch through the early 1990s was just 6 birds, but 20 were observed between April–June 1996, 11 were seen 28 March 2008, and 16 were counted on both 4 April 2010 and 16 April 2012; 0 were seen at Alisal Ranch in 2013; but up to 10 birds, 4 nests, and 9 young were there in 2016, 4 birds and nests were found 18 July 2018, but perhaps only 1 nest 9 June 2020. These birds in District I primarily utilize holes in large Western Sycamores for nesting. The lowland breeding birds may arrive as early as mid-March (e.g., 11 March 1975 Nojoqui Falls County Park (4), 12 March 2017 Nojoqui Falls County Park (2), 13 March 1991 Lake Cachuma), although typically they are not seen until late that month. They probably depart the breeding areas by mid-August, though 15 birds were still present 12 August 2014 and 4 were there 13 August 2017.
There also have been several relatively recent late-spring/summer sightings elsewhere in the Santa Ynez Valley, which suggest sporadic local nesting in these areas as well. These records are: 2 near Los Olivos during 1980; 1 near Santa Ynez in 1980 and 1981; 2 near Los Olivos in early July 1984; 4 near Santa Ynez 9 July 1986, 6 there 28 June 1992, 2 there (Happy Canyon Road) 18 May 2015, and 4 (2 adults and 2 juveniles) present (Quiota Creek) 24 July 2016.
Purple Martins also nested into the 1980s in the San Rafael Mountain, Big Pine Mountain, and Don Victor Valley areas in District M. One pair had a nest in a dead Jeffrey Pine at the summit of Big Pine Mountain during July 1979, and 2 pairs frequented a large, dead pine on West Big Pine Mountain 1 July 1981; none has been seen there during subsequent visits. Fourteen individuals were found nesting in dead Ponderosa Pines in the Mission Pine Basin area of San Rafael Mountain 18–19 June 1982 and 6 were there 10–12 June 1989. Six were in the Don Victor Valley east of Big Pine Mountain 29 April 1981 and at least 3 were there 3–15 May 1982. One at East Pinery near Figueroa Mountain 5 July 2015 and 4 at Figueroa Mountain 12 May 2019 might have wandered up from the Santa Ynez Valley.